The invention relates to a television camera tube comprising in one end of an evacuated tubular envelope an electron gun to generate an electron beam, which electron beam is deflected so that it describes a frame on a photoconductive target, which target is provided on or near the inside of a window closing the other end of the envelope, against the outside of which window an antihalo plate is provided the circular surface of which has a reduced light reflexion.
Examples of such tubes are vidicon tubes, Plumbicon (Registered Trade Mark) tubes, SEC tubes and tubes having a hetero junction layer, for example, Zn-Cd-Te. The target in such tubes usually consists of a transparent signal electrode which is provided on the window and on which a photoconductive layer is provided. The operation of such tubes is as follows. The signal electrode is connected to a voltage source via a signal resistor. The potential of the signal electrode is positive relative to the potential of the cathode of the electron gun which is at zero volts. The scene to be picked up is projected onto the photoconductive layer through the transparent signal electrode. The side of the photoconductive layer facing the electron gun may be considered as being composed of a large number of picture elements. Under the influence of the positive potential of the signal electrode the potential of said picture elements locally increases as a result of photoconduction of the photoconductive layer. Thus the potential of the picture elements depends on the incident light intensity. As a result of this a potential picture is formed on the side of the photoconductive layer facing the electron gun. Since the electron beam is deflected and describes a frame on the target, the potential of the picture elements is reduced periodically to the potential of the cathode. The current associated therewith flows via the signal electrode which all picture elements have in common, through the signal resistor as a result of which a voltage is formed across said signal resistor. This voltage represents, as a function of time, the light intensity of the optical image as a function of the place on the photoconductive layer. In such tubes a target is usually used which covers substantially the whole window.
In such a television camera tube which is disclosed in Netherlands Patent Application No. 7603829 (PHN 8357) laid open to public inspection, a phenomenon referred to as "flare" often occurs, which disturbs the picture and which is caused by overexposure of specific parts of the target by stray light. This stray light is formed by internal reflections both in front of and behind the target. Flare can be reduced inter alia by means of the antihalo plate which is provided against the outside of the window immediately in front of the target. Additionally the antihalo plate ensures that dust particles which would otherwise become visible when the lens of the camera has a small aperture cannot come in the focal plane. In Netherlands Patent Application No. 7603829 laid open to public inspection it is described that the peripheral surface of the antihalo plate consists of a non-reflective material and thus partly suppresses the undesired reflections. However, this measure proves to be insufficient. Of the light which passes through the camera lens and the antihalo plate onto the target a part is absorbed and produces the photosignal, another part is passed through, and a further part is reflected by the target. In the case of red light, the passed and reflected light causes strong flare so that discolouring of the picture over the whole surface may result. For red light the ratio of absorbed-passed-reflected light is approximately 1:1:1. The flare caused by the light passed through the target is controlled effectively in known manner by blackening the mesh which, viewed from the antihalo plate, is present immediately behind the target. However, the reflected red light still produces problems, in particular in the case of large angles of incidence.
The image produced by the camera lens often extends over the whole surface of the antihalo plate which is much larger than the frame which is described by the electron beam. A mask on the entrance face of the antihalo plate is of little effect and/or gives rise to so-called "vignetting" (this is a strong brightness decline towards the edge of the picture). Vignetting is also produced when a rectangular antihalo plate having slightly larger dimensions than the dimensions of the frame is provided.